Large combinatorial libraries of biopolymers are starting points for isolating new enzymes, binding motifs and other useful molecules. For example, current technologies can generate populations of nucleic acids with complexities on the order of 10.sup.15 molecules and then isolate and identify a single molecule with a desired activity. Random polypeptide populations have greater chemical diversity than do polynucleotides, making them an attractive alternative to nucleic acids. Current systems are limited in their ability to easily generate large complex libraries of polypeptides that are in a form that allows the isolation and identification of rare molecules with a desired activity.